fix: null-safety and config validation for aws.s3.toggles source

[Copilot]

Three issues flagged in review on the aws.s3.toggles source: missing config validation causing opaque NPEs, and unsafe auto-unboxing of a nullable Property<Boolean> in the Spring demo.

Changes

• CoreSourceFactory – Validate bucketName and togglesFolder before building S3TogglesSource; throw IllegalArgumentException with the missing key name instead of letting Lombok @NonNull or asS3Folder fail silently with no context:

  String bucketName = context.getConfiguration().getValue("otto.config.aws.s3.toggles.bucket.name");
  if (bucketName == null) {
      throw new IllegalArgumentException("Missing required configuration key: otto.config.aws.s3.toggles.bucket.name");
  }

• demo/spring/DemoService – Replace auto-unboxing s3Toggle1.getValue() ? … with Boolean.TRUE.equals(s3Toggle1.getValue()) ? … to avoid NPE when the toggle is absent. Matches the pattern already used in the Helidon demo.

• CoreSourceFactoryTest – Two new tests assert IllegalArgumentException is thrown when bucketName or togglesFolder is missing from configuration.